reclaimed water use in agriculture: effects on soil chemical
TRANSCRIPT
Citation: Leogrande, R.; Pedrero, F.;
Nicolas, E.; Vitti, C.; Lacolla, G.;
Stellacci, A.M. Reclaimed Water Use
in Agriculture: Effects on Soil
Chemical and Biological Properties in
a Long-Term Irrigated Citrus Farm.
Agronomy 2022, 12, 1317. https://
doi.org/10.3390/agronomy12061317
Academic Editors:Andrea Baglieri
Received: 31 March 2022
Accepted: 27 May 2022
Published: 30 May 2022
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agronomy
Article
Reclaimed Water Use in Agriculture: Effects on Soil Chemicaland Biological Properties in a Long-Term Irrigated Citrus FarmRita Leogrande 1,* , Francisco Pedrero 2 , Emilio Nicolas 2, Carolina Vitti 1, Giovanni Lacolla 3
and Anna Maria Stellacci 4
1 Council for Agricultural Research and Economics, Research Centre for Agriculture andEnvironment (CREA-AA), 70125 Bari, Italy; [email protected]
2 Centro de Edafología y Biología Aplicada del Segura (CEBAS-CSIC), Irrigation Department,Campus Universitario de Espinardo, 30100 Murcia, Spain; [email protected] (F.P.);[email protected] (E.N.)
3 Department of Agricultural and Environmental Science (DiSAAT), University of Bari, 70126 Bari, Italy;[email protected]
4 Department of Soil, Plant and Food Sciences (DiSSPA), University of Bari, 70126 Bari, Italy;[email protected]
* Correspondence: [email protected]
Abstract: In Mediterranean regions, the scarcity of freshwater for agricultural purposes is leading tothe use of alternative water sources. This study aimed to evaluate the effects of long-term irrigationwith reclaimed water on chemical and biological soil properties. On a mandarin tree orchard (Citrusclementina, cv. Orogrande), freshwater (FW) and tertiary reclaimed water (RW) were supplied forirrigation. In spring 2017, a soil sampling was carried out, collecting from each experimental plotfour samples at 0–0.20 m depth. Chemical and biochemical soil properties were determined on airdried and sieved soil and on fresh and field-moist soil, respectively. The irrigation with reclaimedwater significantly increased the soil water extractable organic carbon (WEOC), available P, Mg, andNa content, and the electrical conductivity (EC). Although not significant, the respiration rates andenzymatic activities were higher in RW treatment. The results of this research highlighted that theirrigation with reclaimed water, providing organic carbon and other nutrients, could have, in thelong-term, beneficial effects on soil microorganism and their activities. In any case, especially in aridand semi-arid environments, a proper management of wastewater should be recommended to avoidsoil degradation due to salt accumulation in the rootzone.
Keywords: tertiary reclaimed water; soil sampling; WEOC; soil respiration; soil enzymatic activities
1. Introduction
The re-use in agriculture of reclaimed water originating from wastewater treatmentplants is becoming an opportunity to increase the water resources for irrigation and a wayto protect the environment from direct disposal of wastewater to the water bodies.
Globally, agriculture is a major consumer of wastewater. The FAO reported thatapproximately 10% of the total global irrigated land area receives untreated or partiallytreated wastewater, encompassing 20 million hectares in 50 countries [1].
Depending on the quality level of the wastewater reclamation treatment and wastewa-ter origin, the irrigation with reclaimed water could differently affect soil properties and,consequently, crop production [2–6]. The impact of the irrigation with this alternative waterresource depends, in fact, on the quality and quantity of wastewater applied, soil type,duration of irrigation, and local climate [6,7].
Considering the growing competition between agriculture and domestic use of freshwater, especially in arid and semiarid environments, and the consequent increasing use ofreclaimed water for irrigation, in order to meet crop water needs, research on long-termeffects of application of reclaimed water on soil properties is of great relevance and interest.
Agronomy 2022, 12, 1317. https://doi.org/10.3390/agronomy12061317 https://www.mdpi.com/journal/agronomy
Agronomy 2022, 12, 1317 2 of 13
Usually, soil physical and chemical variables (such as pH, texture, bulk density, waterretention capacity, soil organic matter, and nutrient availability) alter very slowly or onlywhen the soils undergo a drastic change. Therefore, investigations focused only on these soilproperties could not yield conclusive evidence of the impacts of irrigation with reclaimedwater, whereas more appropriate variables should be considered in order to assess theeffects of reclaimed water on soil quality and health and on the ecosystem functions.
Several studies highlighted that irrigation with reclaimed water could affect the or-ganic matter and essential crop nutrient content in soils [2,4,8,9] and, consequently, couldpromote soil aggregate stability, water retention capacity, cation exchangeable capacity, andthe microorganism activities and structure. Tarchouna et al. [7] observed that, althoughwastewater contained organic carbon (OC), the supply of wastewater diminished OCcontent in soil, probably related to an intensification of microbial activity due to labile Cand N supplied by the wastewater. Labile and biodegradable fraction of soil total organicmatter (estimated as water extractable organic C, WEOC) is highly related to the plantnutrient cycling and to soil microbial activity, since it is an immediate energy source formicroorganisms [10,11]. In addition, this fraction is highly sensitive to tillage, manuring,fertilization, crop rotation, and other agronomic management in comparison with total or-ganic matter [12–15]. It is consequently considered, along with soil respiration and enzymeactivity, a sensitive indicator of management-induced changes in soil quality [16–21].
Soil respiration and enzymatic activities are good estimators of overall biologicalactivity, and they are closely related to the processes of organic matter decomposition andmineralization [22–26]. Previous studies showed that numerous factors such as climate,type of amendment, agricultural management, crop type, and edaphic properties can affectthese sensitive soil indicators [27–33]. Consequently, wastewater, being a potential sourceof chemical substances applied to the soil, may change gradually the microbial dynamics.
Since the soils are living systems containing vast assemblages of soil organisms whichperform critical functions to terrestrial life, sensitive soil variables such as WEOC, res-piration, and enzymatic activities should be considered when investigating the effect ofmanagement practices on soil health.
In light of the issues reported, a study was carried out to assess the effects of long-termirrigation with reclaimed water on main chemical and biochemical soil properties, with aparticular focus on labile and biodegradable fractions of soil organic matter, soil respiration,and enzymatic activities as sensitive indicators of management-induced modifications ofsoil quality status. To this aim, in a mandarin (Citrus clementina, cv. Orogrande) orchard inMurcia region (Spain), irrigated over ten consecutive years with tertiary reclaimed water, asoil sampling was carried out in spring 2017 when soil moisture and temperature conditionsshould be favorable for microbial activities.
2. Materials and Methods2.1. Experimental Site and Treatments
The long-term field trial was established in 2008 on an orchard, located in Campotéjar-Murcia, Spain (38◦07′18′′ N; 1◦13′15′′ W), characterized by a Mediterranean semiaridclimate. The orchard was cultivated with fully-grown mandarin trees (Citrus clementina,cv. Orogrande) with tree spacing of 5 m × 3.5 m. The historical annual reference evapo-transpiration (ETo) and rainfall (2008–2017) are, on average, 1365 and 284 mm, respectively(Figure 1). 2017 was a very dry year in comparison with historical data, with a total rainfallof 204 mm [34]. The soil was classified as a sandy clay loam (USDA classification), withaverage contents in sand, silt, and clay of 42%, 32%, and 26%, respectively. In 2007, beforethe beginning of applying reclaimed water, the soil electrical conductivity of the saturatedpaste extract (ECe) was, on average, 2.1 dS m−1 [35].
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Figure 1. Monthly average reference evapotranspiration (ETo), rainfall, and maximum and minimumtemperatures (TMAX, TMIN) recorded during 10 years of experiment (2008–2017).
The long-term field experiment compared two water sources and two irrigation treat-ments. The two water sources were: water transferred from the channel (FW), with anaverage electrical conductivity of 1.2 dS m−1, and tertiary reclaimed water (RW) pumpedfrom the Molina de Segura tertiary wastewater treatment plant with an average electricalconductivity of 3.4 dS m−1 [34]. The average chemical composition of the two water sourcesused for irrigation is reported in Table 1.
Table 1. Average chemical composition of freshwater (FW) and reclaimed water (RW) used inexperimental trial (ECw, electrical conductivity; SARw, sodium adsorption ratio; pH and the contentof cations and anions.
Parameters Units FW RW
ECw dS m−1 1.2 3.4SARw 2.2 6.4
pH 8.2 7.8Ca2+ meq L−1 4.1 6.8Mg2+ meq L−1 4.0 8.2
K+ mg L−1 9.1 36.1Na+ meq L−1 4.6 16.8Cl− meq L−1 3.9 15.4
NO−3 mg L−1 3.8 12.2PO3−
4 mg L−1 1.8 2.2SO2−
4 meq L−1 5.4 12.4Modified from Abadia et al. [34].
The two irrigation treatments were: irrigation to fully satisfy the crop water require-ments (100% of actual crop evapotranspiration, ETc) and regulated deficit irrigation (RDI),restoring 100% of ETc during the whole year, except during the second stage of fruit de-velopment, phenological periods less sensitive to water stress, when only the 50% of ETcwas restored [34]. Irrigation was scheduled on the basis of weekly ETc estimated as cropreference evapotranspiration (ETo), calculated using the Penman–Monteith equation [36],
Agronomy 2022, 12, 1317 4 of 13
and month-specific crop factors [37], as described by Pedrero et al. [35]. The meteorologicaldata were collected from an automated weather station situated in the experimental field.The average amounts of water applied in the full irrigation and in RDI treatments were7500 and 6200 m3 ha−1, respectively. Hence, RDI treatment allowed average reductions ofabout 17% of water per year. Both treatments included application of the same amounts offertilizer (N—P2O5—K2O) applied through the drip irrigation system.
The experimental trial was arranged in a randomized complete block design (RCBD)with four replications. The single plot size was of 210 m2 with twelve mandarin trees.In this study, only the regulated deficit irrigation treatments with and without reclaimedwater were considered.
2.2. Soil Sampling and Laboratory Analysis
In spring 2017, after ten years of irrigation with reclaimed water, soil characterizationwas performed, collecting from each plot four soil samples at 0–0.20 m depth, on 35 m2 testarea placed in the middle row of the plot. Each sample was divided into two subsamples:one was immediately stored at 4 ◦C in plastic bags until assaying of biochemical analysis,the other was air-dried and sieved for chemical one.
On air-dried and sieved soil sampling, total organic carbon content (TOC) was mea-sured by dry combustion [38] with a TOC Vario Select analyzer (Elementar, Hanau, Ger-many), total nitrogen (N) was analyzed according to the Kjeldahl procedure, availablephosphorus (P) was determined according to Olsen method; exchangeable potassium(K), calcium (Ca), magnesium (Mg), and sodium (Na) were determined by extraction in abarium chloride–triethanolamine buffered solution (pH = 8.2), followed by Inductively Cou-pled Plasma-Optical Emission spectrometry (ICP-OES) quantification [39]; pH and electricalconductivity (EC) were measured in a 1/5 (w/v) aqueous/soil extract, pH was quantifiedwith a CRISON microTT 2050 pHmeter and EC1:5 with a CRISON GPL32 conductivimeter.To convert the EC1:5 values to ECe, the following equation was applied [40]:
ECe = EC1:5 × 7.5 (1)
Exchangeable sodium percentage (ESP) was calculated as follows [41]:
ESP (%) =Na+
CEC× 100 (2)
where Na+ is the ion concentration on exchange complex (meq 100 g−1) and CEC is thesoil cation exchangeable capacity (meq 100 g−1), calculated as the sum of the exchangeablecation concentrations (Ca2+, Mg2+, K+, and Na+) [41].
On fresh and field-moist soil samples, water extractable organic carbon (WEOC) wasmeasured. Extraction of water-soluble carbon from the soil was carried out accordingto a protocol obtained by combining procedures reported in Haynes [42] and Rees andParker [43]: 30 g of fresh-weight soil and 60 mL of distilled water (1:2 solid/liquid ratio)was placed in an Erlenmeyer flask, capped, and shaken for 30 min. The extracts werecentrifuged at 10,000 rpm for 10 min and the supernatant was filtered through 45 µmMillepore filter. Total organic carbon in water extracts was analyzed with TOC Vario Selectanalyzer (Elementar, Germany) [44].
On two out of the four fresh samples per plot, soil respiration and enzymatic activities(dehydrogenase, urease, alkaline phosphatase, and β-glucosidase) were quantified. Soilrespiration was measured by the titrimetric method [45]. This method is based on thedetermination of CO2 evolution from incubated soil. Approximately 30 g of soil was placedin vials, and deionized water was added to obtain a gravimetric water content of 30% [46].Each vial containing soil was placed in a 1.0 L glass jar containing a vial with 4 mL of 1-MNaOH; moreover, 4 mL of acidified water was added into the base of the jar to maintainthe humidity and reduce soil drying. Each jar was sealed tightly and placed in darknessin an incubation chamber with constant temperature of 28 ◦C. During incubation the CO2
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produced was trapped into NaOH solution, forming Na2CO3, and the excess NaOH, whichdid not react, was titrated with HCl. Soil respiration was determined after 1, 3, 7, 14, 21,and 28 days of incubation: the glass jar was opened and the reaction of hydroxide withcarbon dioxide was immediately stopped by adding 8 mL of 0.75-N BaCl2, then the NaOHvial was removed from the jar, and the excess NaOH was titrated out with 0.1 M HCl,setting the titration pH of 8.8. Soil respiration as µg C–CO2 g−1 h−1 was computed fromthe titration value according to the equation:
C− CO2 =(V0 −V)×M× EW × 1000
dw× hinc(3)
where V0 is the volume (mL) of acid (HCl) used to titrate the alkali (NaOH) of a blanksolution (incubated without soil), V is the volume (mL) of acid used to titrate the alkalisolution incubated with the soil sample, M is the molarity (mol L−1) of the acid (HCl) andEW is the equivalent weight (g mol−1) of C–CO2, 1000 is the converting factor from mgto µg, dw is the dry weight of soil (g), and hinc is the incubation duration expressed inhours [47].
Dehydrogenase activity was determined by the method of Masciandaro et al. [48].In this procedure, 1 g of soil was mixed with 0.2 mL of 4% INT (2-(4-Iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride) in distilled water. The control was the soiltreated with distilled water (0.2 mL), instead of INT. Soils were incubated for 20 h at 22 ◦C indarkness. The INTF (1-(4-Iodophenyl)-5-(4-nitrophenyl)-3-phenylformazan) formed by thereduction of INT was extracted by adding 10 mL of a mixture of 1:1.5 tetrachloroethyleneand acetone; next, the soil mixture was vigorously shaken by hand for 1 min and finallyfiltered through a Whatman no. 41 filter paper. The INTF concentration was measuredspectrophotometrically at 490 nm, and the results were expressed as µg INTF g–1 soil h–1.
Urease activity was determined by the buffered method of Kandeler and Gerber [49].In this procedure, 2.5 mL of a solution of urea (0.72 M) and 20 mL of borate buffer (pH 10)were added to 5 g of soil in hermetically sealed flasks, and then incubated for 2 h at37 ◦C. Then, the reaction was stopped by adding 30 mL of a mix solution of KCl/HCl(1 N/0.01 N) and the ammonium content was determined according to the modified Berth-elot’s method [50]. Controls were prepared without substrate to determine the ammoniumproduced in the absence of added urea. The intensity of green color that forms upontreatment of an aliquot of the extract with salicylate and hypochlorite at high pH wasdetermined at 667 nm in a spectrometer, and the enzymatic activity was expressed asµg NH3 g–1 soil h–1.
The alkaline phosphatase and β-glucosidase determination follow similar protocols.Alkaline phosphatase (Alkaline phosphomonoesterease) activity was measured by themethod of Tabatabai and Bremner [51], adding 0.2 mL of toluene, 4 mL of MUB (modifieduniversal buffer, pH 11), and 1 mL of substrate 0.025 M p-nitrophenyl phosphate (p-NPP) to1 g of soil. β-glucosidase activity was determined according to a modification of Tabatabai’smethod [52], adding 0.2 mL of toluene, 4 mL of MUB, and 1 mL of substrate 0.025 M ofp-nitrophenyl-β-D-glucopyranoside (pNG) at 1 g of soil. The mixture obtained is thenincubated at 37 ◦C for one hour. Following incubation, 1 mL 0.5 M of CaCl2 and 4 mL 0.5 Mof NaOH were added for the alkaline phosphomonoesterease activity assay, and 4 mL oftris(hydroxymethyl)aminomethane-sodium hydroxide (THAM-NaOH) was added for theβ-glucosidase assay. For the control test, the respective substrate was added before theaddition of CaCl2 and NaOH (i.e., immediately before the soil suspension is filtered). Theabsorbance of the supernatant from the reaction (p-nitrophenol) was determined at 400nm in a spectrometer and the enzymatic activities were expressed as µg p-nitrophenol g–1
soil h–1.The results of enzymatic activities are expressed on soil dry weight.
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2.3. Data Analysis
Soil data were preliminary analyzed in order to investigate the distribution of thevariables under study for the whole data set. Data were then tested for normality usingthe Kolmogorov–Smirnov test. This preliminary evaluation showed that, for most of thevariables considered, data distribution did not deviate from Gaussianity according to thenormality test.
Then, data were subjected to a nested analysis of variance for a randomized com-plete block design (RCBD), considering the sub-replicates within each plot as pseudo-replicates [47]; means were compared using SNK post hoc test at p = 0.05 level. Moreover,Person linear correlation analysis was performed to investigate relationships betweenchemical and biological soil variables. Data analysis was carried out using the SAS/STAT9.2 software [53].
3. Results and Discussion
The irrigation over ten consecutive years with reclaimed water affected soil chemicalproperties (Table 2). In particular, although not significant, total organic carbon (TOC)content was higher in the treatment RW (+28%) than FW Previous studies reported con-flicting results related to the TOC increase in soil irrigated with reclaimed water. In facts,Rusan et al. [2] observed, in a soil characterized by fine texture, increases of organic mattermostly in the topsoil after two years of application, and Bedbabis et al. [4] found that in asandy soil the organic matter content significantly increased in the long-term application.Conversely, other authors highlighted decreases of organic matter, especially in coarsesandy soil, probably due to supplied nutrients by irrigation with wastewater that stimu-lated microbial activity in the soil, promoting mineralization of organic matter [7,54,55].These different results can depend on several factors such as wastewater composition,period of application, soil texture and management, etc. In our study, the irrigation withreclaimed water significantly increased the soil water extractable organic carbon (WEOC)content (Table 2); these results agreed with previous research [7]. Therefore, the applicationof reclaimed water can be a strategy to save freshwater resources and increase soil organicmatter content, mostly its labile fraction, that represents an immediate energy source formicroorganisms involved in several soil biological processes (organic matter decomposition,nutrients cycling, etc.) [12].
Table 2. Effects of different water quality on soil chemical properties (FW = freshwater; RW = reclaimedwater).
Treatments TOC WEOC pH EC1:5 Total N Available P Ca2+ K+ Mg2+ Na+ ESPg kg−1 mg kg−1 dS m−1 g kg−1 mg kg−1 mg kg−1 mg kg−1 mg kg−1 mg kg−1 %
FW 17.004 14.302 8.462 0.481 0.904 154.9 2345.9 268.1 341.9 65.8 1.857RW 21.770 22.514 8.486 0.717 1.024 241.5 2515.8 235.2 518.4 358.3 8.356
n.s. * n.s. ** n.s. ** n.s. n.s. * *** ***
*, **, ***= Significant at the p ≤ 0.05, 0.01 and 0.001 levels, respectively; n.s. = not significant.
The pH and N, Ca and K content were not affected by treatments under study, eventhough a slight increase was observed for N and Ca contents (+13 and +7%, respectively) inthe soil irrigated with reclaimed water. This finding disagrees with other previous studiesthat showed significant increases of soil pH, attributed to the high content of cations suchas Ca, Mg, and Na [4,7].
In our study, soil EC, available P, Mg2+, Na+, and ESP were significantly higher in RWtreatment (Table 2). In particular, EC1:5, after ten years of irrigation, was significantly higher(+49%) in RW treatment compared with FW (Table 2). This behavior was predictable consid-ering the chemical characteristics of the reclaimed water (Table 1). In any case, the irrigationpractice increased the soil salinity, regardless of the type of water used. In fact, comparedwith the soil EC value at the beginning of the experimental trial (ECe = 2.1 dS m−1), averageincreases of 72 and 156% were observed, respectively, for FW and RW treatments. Afterten years of irrigation with reclaimed water, the soil electrical conductivity (5.38 dS m−1),
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expressed as EC of the saturated paste extract (ECe = EC1:5 × 7.5, [40]), exceeded the salin-ity threshold value (4 dS m−1), whereas the soil ECe in FW treatment was slightly belowthe threshold value (3.6 dS m−1). Similar results were observed on the same experimentpreviously, where it was shown that irrigation with reclaimed water tends to accumulatesalts in the root zone, therefore, the use of these irrigation waters can cause problems ofsalinity and accumulation of B in the plant and soil in the long-term [56–58]. This findingis also in agreement with other long-term studies, highlighting that salts supplied withreclaimed water could accumulate in the root zone and increase soil salinity [2,4,59] withadverse consequences for soil quality and crop yield. In RW treatment, the soil Na contentand ESP value were 5.4 and 4.5 times higher than FW. However, due to high exchangeableCa and Mg soil content, after ten years of wastewater irrigation the soil cannot be definedsodic because the ESP value remained lower than the soil sodicity threshold (15%). Fur-thermore, the high soil concentrations of Ca and Mg, due to the wastewater application(Table 1), could counteract the negative effects of the Na on the soil particle dispersion.In other studies, it was observed that long-term irrigation with reclaimed water causedimportant increases of soil ESP values [5,59], also far above the soil sodicity threshold. Highlevels of soil sodicity obtained after irrigation with wastewater could lead to clay particlesdispersion, aggregate destabilization, impaired water movement in the soil profile, aerationproblems, and salt accumulation [59]. The composition of the water, particularly the cationratios, can affect soil aggregate stability, which in turn affects soil physical conditions andwater infiltration [6]. The negative impact of sodicity is more evident in soils with highclay contents [60,61]. Physical and chemical properties, particularly of high clay soils, havebeen found to be degraded following long-term irrigation with reclaimed water [62–64].In our experiment, the soil could be probably considered with low sensitivity to sodicityeffect because clay content was, on average, 26%. In any case, based on these results, it isvery important to adopt proper management of reclaimed water and periodic monitoringof soil properties to avoid soil degradation.
Soil respiration, estimated as the CO2 produced and released during an incubationperiod of 28 days, did not show significant differences between treatments (Figure 2). Theresults indicated that the highest C–CO2 emission rates were found at the beginning ofthe experiment, then they tended to decline with incubation time. The rate of C–CO2emission from soil reached a maximum after one day of incubation, with values of 14.8and 9.8 µg g−1 h−1 in RW and FW treatments, respectively. These values dropped to 4.5and 3.0 µg g−1 h−1, in RW and FW, respectively, at the end of incubation time (after28 days). In both treatments, higher availability of the readily labile organic C fraction tosoil microorganisms at the beginning of the incubation period increased soil respiration ratewhich gradually decreased during the experiment, probably due to the gradual depletionof the available pool of C [65,66].
In RW treatment, characterized by the highest values of WEOC and TOC, higher soilrespiration rates were observed during the whole period of incubation. Likely, the soilmicrobial activity in the RW treatment was stimulated by the greater organic matter avail-ability, which created favorable conditions for soil microorganisms and consequently soilrespiration and C–CO2 emissions increased [66]. Linear correlation analysis showed thatsoil TOC and WEOC content were significantly and positively correlated with respirationrate in the whole period of soil incubation (Table 3). In particular, a stronger relationshipwas observed between respiration rates in the whole incubation period and WEOC, thelabile fraction of TOC (significance of Pearson’s correlation ranged from 0.0002 to <0.0001),whereas it was weaker with total organic carbon. Similar results were found in previousstudies, highlighting that the quality of organic matter affects CO2 fluxes more than thequantity [67,68]. In addition, the soil total nitrogen content was positively and significantlycorrelated with soil respiration rate in the whole incubation period (Table 3), as observedalso by Fan et al. [69], who found that the respiration rate mainly depended on organicsoil C and N content. Consequently, in our research, the higher soil N content in the RW
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treatment could have contributed to increase the respiration rate in this treatment comparedto FW one.
Figure 2. Soil respiration rate during 28 days of incubation in the two different quality watertreatments (FW, freshwater; RW, reclaimed water).
Table 3. Pearson’s linear correlation coefficients between the main soil chemical properties andrespiration rate and enzymatic activities: in brackets the number of samples used is reported.
Soil Respiration Enzyme Activities
1 Day(34)
3 Days(36)
7 Days(36)
10 Days(36)
14 Days(36)
21 Days(36)
28 Days(36)
Urease(36)
Dehydr(36)
βGluc(36)
Phosph(36)
TOC 0.45978 0.59437 0.61466 0.64113 0.61961 0.62516 0.67392 0.23013 0.51759 0.41802 −0.012070.063 0.0119 0.0086 0.0055 0.008 0.0073 0.003 0.3742 0.0333 0.095 0.9633
WEOC 0.69787 0.78623 0.85926 0.83792 0.89064 0.9052 0.88855 0.69849 0.77609 0.4382 0.41590.0018 0.0002 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.0018 0.0002 0.0785 0.0968
pH −0.3959 −0.32745 −0.30456 −0.26701 −0.23229 −0.27723 −0.31231 −0.39116 0.08072 −0.13862 −0.144040.1157 0.1995 0.2346 0.3002 0.3696 0.2814 0.2223 0.1205 0.7581 0.5957 0.5813
EC1:5 0.48219 0.56918 0.55966 0.59199 0.53791 0.57089 0.58783 0.43388 0.31287 0.2756 0.315760.05 0.0171 0.0195 0.0123 0.0259 0.0167 0.0131 0.818 0.2214 0.2843 0.217
Available P 0.1986 0.24264 0.34859 0.38744 0.35139 0.35608 0.38532 0.32302 0.47847 0.20872 0.063690.4448 0.3481 0.1703 0.1244 0.1666 0.1607 0.1267 0.206 0.052 0.4214 0.8081
Total N 0.71939 0.79253 0.8092 0.8236 0.76879 0.75451 0.78736 0.3378 0.67907 0.80982 0.167810.0011 0.0001 <0.0001 <0.0001 0.0003 0.0005 0.0002 0.1848 0.0027 <0.0001 0.5197
Ca2+ 0.3419 0.44895 0.47784 0.4816 0.46576 0.48694 0.54279 0.17266 0.31619 0.23773 −0.096450.1792 0.0706 0.0524 0.0503 0.0595 0.0474 0.0244 0.5075 0.2163 0.3582 0.7127
K+ 0.1597 0.10658 0.0958 0.03214 0.02285 0.06141 0.11375 0.07441 −0.18581 −0.06115 −0.104840.5404 0.6839 0.7145 0.9025 0.9306 0.8149 0.6638 0.7765 0.4752 0.8157 0.6888
Mg2+ 0.59715 0.70762 0.65355 0.68948 0.65998 0.6585 0.64061 0.29127 0.50528 0.36482 0.296560.0114 0.0015 0.0044 0.0022 0.0039 0.004 0.0056 0.2567 0.0386 0.1499 0.2477
Na+ 0.48417 0.59072 0.55174 0.61125 0.57504 0.5743 0.53763 0.29294 0.53597 0.30568 0.375550.0489 0.0125 0.0217 0.0091 0.0157 0.0159 0.026 0.2538 0.0266 0.2328 0.1374
ESP 0.3802 0.46019 0.42087 0.4885 0.45315 0.44308 0.38901 0.26508 0.47294 0.25072 0.41130.1322 0.0631 0.0925 0.0466 0.0677 0.0749 0.1228 0.3038 0.0552 0.3317 0.101
The p values are reported in italics.
All soil enzymes measured exhibited higher activity at the RW treatment than FW(Figure 3). Chen et al. [70] demonstrated that long-term irrigation with reclaimed waterdid not adversely affect soil enzyme activities; on the contrary, in particular conditions, itstimulated them. The increases of enzymatic activities, especially that involving nutrient
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and C cycle, were probably due to the accumulation in the soil of biodegradable organicC with the application of wastewater. In our study, significant and positive correlationswere found between WEOC values and urease and dehydrogenase activities (Table 3),proving that soil microbial activities were strongly linked to the quality of organic matter.In addition, the total N content was also significantly and positively correlated withdehydrogenase and βglucosidase activities, as reported by other authors [71].
Figure 3. Soil enzymatic activity in the two different quality water treatments (FW, freshwater; RW,reclaimed water).
If the soil enzyme activities remain unchanged or increase under irrigation withreclaimed water, the biogeochemical processes governing C, N, and P cycling are notinterrupted, and ecosystem functions should be saved. In this manner, the application ofreclaimed water would be environmentally sustainable. The inhibition of the activities ofone or more key enzymes could have significant consequences in the rate of soil nutrientscycling and, consequently, in the long-term, soils irrigated with wastewater could beexposed to degradation risk. Furthermore, irrigation with high quality reclaimed water(tertiary treatment) may minimize the potential adverse effects from toxic and persistentorganic compounds [70].
Results from this field investigation indicated the beneficial effects that, in the long-term period, irrigation with reclaimed water had on soil biological health related to thebiodegradable organic matter and nutrients supplied with reclaimed water, which served asa source of energy for microorganisms and enzymes. Then, agricultural reuse of reclaimedwater can be considered an ecologically viable method to disposal of these waters. In anycase, careful management of wastewater is always recommended to avoid an excessiveaccumulation of salts and other undesirable compounds that can contribute to soil degra-dation. The risks of wastewater reuse in agriculture are extensive, ranging from changesto physico-chemical and biological properties of soils to impacts on human health. Theirrigation with treated wastewater could affect soils differently, depending on intrinsicsoil characteristics, environment conditions, wastewater origin, and quality of reclamationprocess. In particular, in arid and semi-arid environments, characterized by uneven andscarce precipitation, rains could not be sufficient to leach the excess salts. Consequently,the salinization in the root zone could have negative effects on soil properties and crops.
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Previous studies highlighted that soil respiration and enzymatic activities could be inhib-ited by soil enrichment, with toxic compounds applied with reclaimed water [70,72,73]. Inrecent decades, the innovative technologies developed to reclaim wastewater and rigor-ous controls on treated water should ensure a safer use of this alternative water resource.Furthermore, continuous monitoring of soil health is recommended to guarantee that theapplication of reclaimed water does not negatively impact on soil chemical, physical, andbiological properties.
4. Conclusions
Irrigation with tertiary reclaimed water (RW) can be an important strategy to savelimited freshwater resources, especially in arid and semi-arid areas. The safe reuse ofwastewater could reduce the amount of the freshwater used in agriculture and preventpotential contamination of both surface water and groundwater. With regard to the soilchemical properties, the RW irrigation, in the long-term, increased the labile and biodegrad-able fraction of organic carbon (WEOC), and this in turn affected soil respiration andenzymatic activities, being a source of readily available energy for soil microorganisms.After 10 years of irrigation with two types of water, significantly higher values of electricalconductivity (ECe) and Na content were observed in the soil receiving reclaimed water(5.4 dS m−1 and 358 mg kg−1, respectively) in comparison to fresh water (3.6 dS m−1 and66 mg kg−1, respectively). Soil irrigated with reclaimed water could be defined as saline(ECe > 4 dS m−1) but not sodic (ESP < 15%), although the Na concentration was about5-fold higher in RW treatment soils than FW ones. The results of this study highlighted thatthe supply of reclaimed water could be a viable strategy, in a view of sustainable use andconservation of natural resources. In any case, when using this water source, monitoringof the main soil chemical and biological properties is always recommended to avoid soildegradation and, consequently, productivity losses.
Author Contributions: Conceptualization, R.L.; methodology, R.L. and A.M.S.; formal analysis,R.L. and C.V.; investigation, R.L., A.M.S., F.P. and E.N.; resources, F.P. and E.N.; data curation, R.L.;writing—original draft preparation, R.L.; writing—review and editing, R.L. and A.M.S.; visualizationG.L.; supervision, R.L.; project administration, A.M.S.; funding acquisition, A.M.S. All authors haveread and agreed to the published version of the manuscript.
Funding: The research was funded by EU and MIUR in the frame of the collaborative internationalconsortium DESERT (ID−217) financed under the ERA-NET Cofund WaterWorks2014 Call. ThisERA-NET is an integral part of the 2015 Joint Activities developed by the Water Challenges for aChanging World Joint Programme Initiative (Water JPI).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available upon request from thecorresponding author (R.L.).
Acknowledgments: The authors would like to thank the EU and MIUR for funding the presentresearch in the frame of the collaborative international consortium DESERT (ID−217) financed underthe ERA-NET Cofund WaterWorks2014 Call. The Authors also thank Marcello Mastrangelo for hisskillful technical assistance in chemical analyzes carried out in the laboratory of the CREA-AA.
Conflicts of Interest: The authors declare no conflict of interest.
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